1. Field of the Invention
The present invention relates to a control device for an internal combustion engine in which the phase of an intake camshaft and/or an exhaust camshaft can be changed relative to a crankshaft and in which NOx produced in lean-burn operation is occluded by a NOx occlusion catalytic converter, and more particularly to a control device for internal combustion engines for performing a so-called rich spike control or controlling to temporarily enrich an air-fuel ratio used in the lean-burn operation so as to thereby purge the NOx occlusion catalytic converter of NOx occluded therein.
2. Description of the Related Art
Conventionally, a control device for an internal combustion engine of this type is known which is described in JP-A-10-82333. The internal combustion engines comprises a catalytic converter, fuel injectors and a cam phase changing mechanism for advancing or retarding the phase of an intake camshaft relative to a crankshaft. The control device controls the internal combustion engine so as to perform a lean-burn operation within a predetermined area and also controls the operation of the cam phase changing mechanism so as to advance or retard the intake valve opening and closing timings in response to the operation of the internal combustion engine. The catalytic converter is a combination of a NOx occlusion catalyst for occluding NOx in exhaust gas resulting from lean-burn operation and a normal three-way catalyst for reducing NOx in exhaust gas resulting from the operation of the internal combustion engine other than the lean-burn operation.
In general, since the NOx occlusion capacity of a NOx occlusion catalyst is limited, when the lean-burn operation of an internal combustion engine continues a long time, the occlusion capacity of the catalyst is reduced. To cope with this, in the aforesaid control device, a rich spike control is performed in which the air-fuel ratio is enriched by 0.3 seconds every time the lean-burn operation exceeds 30 seconds to thereby reduce the oxygen concentration in the exhaust gas. Thus, the NOx occlusion capacity of the NOx occlusion catalyst is recovered by performing the rich spike control periodically to expel NOx occluded in the NOx occlusion catalyst. On the other hand, NOx expelled from the NOx occlusion catalyst is reduced by the three-way catalyst. Furthermore, in this control device, the cam phase changing mechanism is prohibited from operating when the rich spike control is performed, whereby the phase of the intake camshaft is constructed to be fixed. This is because the cam phase changing mechanism is constructed to operate in a hydraulic fashion, and therefore in a case where the phase of the intake camshaft is changed in response to the rich spike control, the response of the cam phase changing mechanism is low and this causes a delay in response until an actual phase change is initiated, whereby the condition of the exhaust gas and the drivability of the vehicle are prevented from getting worse due to the delayed response.
According to the aforesaid conventional control device for internal combustion engines, since the rich spike control is performed during the lean-burn operation with the phase of the intake camshaft being fixed, as shown in FIG. 6, there is caused torque fluctuation in which the engine torque is temporarily increased in conjunction with enrichment of the air-fuel ratio (A/F) during the rich spike control, whereby the drivability is deteriorated. In addition, as shown in the same figure, the NOx content in the exhaust gas is temporarily increased in conjunction with the enrichment of the air-fuel ratio, and this increases the load borne by the three-way catalyst of the catalytic converter, whereby the amount of NOx is increased which is discharged without being reduced by the three-way catalyst. If the execution interval of the aforesaid rich spike control is shortened with a view to avoiding this, the engine is operated with the enriched air-fuel ratio more frequently, resulting in deterioration in fuel consumption.